The present invention relates to an apparatus for controlling the injection timing of the fuel to be injected into an internal combustion engine.
For the purpose of electronically controlling the timing of the fuel supply to an internal combustion engine in response to the operating conditions of the engine, various types of apparatuses for controlling the timing of fuel injection have been proposed (c.f. SAE 800167). In general, this type of apparatus has a sensor for detecting the timing of a predetermined reference angle position (e.g. top dead center timing) of the crankshaft of the engine and another sensor for detecting the actual timing of fuel injection by a fuel injection pump and it is so arranged that the position of an injection timing adjusting member of the fuel injection pump is controlled in such a way that the actual timing of injection as determined from the outputs from the above-mentioned sensors is coincident with the optimum injection timing at each instant computed on the basis of operation parameters, such as the rotational speed of the engine. The use of a closed loop system as mentioned above can be expected to realize highly accurate control of the timing of fuel injection. However, in the low rotational speed zone of the engine, it brings about a disadvantage which impairs the stable control condition. The reasons for this will be explained below.
In the low rotational speed zone where the amount of injection is rather small, the control conditions become unstable because the signal waveform required for determining the actual injection timing cannot be obtained owing to the low level of the output signal from the sensor for detecting the actual fuel injection timing (for example, a needle valve lift sensor provided in the fuel injection valve). Moreover, excessive amplification of the sensor signal would decrease S/N ratio of the signal and lead to erroneous operation. Further, a large error occurs in the computation of the actual fuel injection timing because of the great changes that occur in the rotational speed within each combustion cycle.
Another disadvantage is that, in driving the injection timing adjusting member by the use of a hydraulic timer or the like, since the oil pressure at low rotational speed is not sufficiently high for the control of the adjusting member, the desired effect cannot be attained even if the closed loop system is employed.